The method uses a quantum cascade infrared laser to strike or “pump” a target sample and a second laser to probe the material’s response. The probe laser reads the absorption spectrum of the molecules present, and the readings can be distilled into pixels that form an image representing the molecules that constitute the sample surface.
The use of a second laser to extract information is a novel aspect of the approach, said Ali Passian of the Measurement Science and Systems Engineering Division (MSSED). “The use of a second laser provides a robust, stable readout approach independent of the pump laser settings.”
Like radar and lidar, the technique uses a return signal to convey information about the substance to be detected, but it differs in important ways, said Passian. It employs a photothermal spectroscopy configuration in which the pump and probe beams are nearly parallel. Probe beam reflectometry provides the return signal in standoff sensing, minimizing the need for expensive wavelength-dependent infrared components such as cameras, telescopes, and detectors.
The findings provide proof of principle for the technique. The work could lead to advances in standoff detectors for use in quality control, airport security, medicine, and forensics. The analysis could be extended to hyperspectral imaging, which provides topographical as well as high-resolution chemical information. “This would allow us to effectively take slices of chemical images and achieve spatial resolution down to individual pixels,” said Passian.
Reference: R.H. Farahi, A. Passian, L. Tetard, T. Thundat. 2012. "Pump-probe photothermal spectroscopy using quantam cascade lasers," J. Physics D 45, 125101.